Question 16
Let $h$ be a semicircle with diameter $AB$. The two circles $k_1$ and $k_2$, $k_1 \neq k_2$, touch the segment $AB$ at the points $C$ and $D$, respectively, and the semicircle $h$ from the inside at the points $E$ and $F$, respectively. Prove that the four points $C$, $D$, $E$ and $F$ lie on a circle.